Tearing fresh herbs, rather than chopping them, helps to prevent bruising and oxidation, which preserves their flavor and appearance. This method also releases their aromatic oils more effectively.
When you chop fresh herbs with a knife, especially if the knife isn't very sharp, you can bruise the delicate leaves. Bruising damages the cell walls, releasing enzymes that cause oxidation. Oxidation leads to browning and a loss of volatile aromatic compounds, diminishing the herb's flavor and visual appeal. Tearing, on the other hand, is a gentler process. It separates the leaves along their natural lines of weakness, minimizing cell damage and preserving the integrity of the herb. Furthermore, tearing releases the herb's essential oils more effectively because it creates a rougher surface area, allowing for greater exposure to air and heat during cooking. This results in a more pronounced and vibrant flavor in your dish. Some herbs, like basil, are particularly susceptible to browning when cut, making tearing the preferred method.
If you must chop herbs, use a very sharp knife and a gentle rocking motion to avoid bruising. Avoid using a dull knife or repeatedly hacking at the herbs.